Each tear is composed of a watery layer sandwiched between an inner layer of mucus and an oily outer layer made of lipids and other fats. Each time we blink, the oily layer blankets the eye, sealing in moisture.
Why Do We Cry?
Researchers identified the substances that make up the oily layer and were surprised to find a class of lipids no one had come across before, called fatty acid amides, said lead author Kelly Nichols, an assistant professor of optometry at Ohio State University.
They also found oleamide, a lipid previously seen only in the brain and central nervous system. Oleamide has been linked to sleep regulation and has also been shown to enable communication between nerve cells. The scientists speculate it may allow for similar communication between cells at the front of the eye.
If there is insufficient or excess oleamide—or any of the other fatty components—the oily lipid layer could lose its ability to keep moisture locked in, resulting in dry eye, Nichols said.